This was taken from Comp Cam's web site: I have always soaked mine in oil but never "pre-pump" them.

It is not necessary to “pre-pump” hydraulic lifters full of engine oil prior to installation and valve
adjustment. It is actually undesirable to do so as the “pumped up” lifters will cause the valves
to open during the adjustment process, rather than positioning the lifter plunger in its operating
position as it is supposed to do. “Pre-soaking” hydraulic lifters in a bath of engine oil is a good
idea but not mandatory. Doing so ensures that the lifters are adequately lubricated on their outer
surfaces prior to installation. It may also result in a quieter engine start up as the oil in the bath
may displace some air from the lifter’s plunger reservoir. Coat the bottoms of all flat tappet